For example, as described in Patent Document 1, a typical hydraulic control device for an internal combustion engine includes a relief valve, which permits some oil to escape into a relief passage when the pressure of the oil discharged by an oil pump become greater than or equal to a predetermined valve opening pressure. In this manner, the pressure of oil supplied to components of the engine is prevented from rising excessively.
A hydraulic control device for an internal combustion engine having a switch valve, which switches the valve opening pressure of the relief valve between, for example, two levels, has been developed. In the hydraulic control device, the switch valve switches the level of the pressure of the oil supplied to the components of the engine between a high pressure level and a low pressure level. Specifically, when, for example, the engine is currently in such an operating state that it is unnecessary to raise the pressure of oil supplied to the components of the engine, the pressure of the oil is switched to the low pressure level, which improves the fuel efficiency.
However, in the hydraulic control device having the switch valve, when the relief valve or the switch valve has a malfunction in which the valve cannot regulate the pressure of oil to the high pressure level, the pressure of the oil supplied to the components of the engine drops. In this case, the engine may not be operated stably when the engine is in such an operating state that the oil under high pressure is necessary. When the relief valve or the switch valve has a malfunction in which the valve cannot switch the pressure of the oil to the low pressure level, the pressure of the oil supplied to the engine components rises excessively, which reduces the fuel efficiency.
This problem also occurs in other hydraulic control devices than the hydraulic control device having the relief valve and the switch valve. That is, a similar problem is caused in any hydraulic control device for an internal combustion engine having a pressure level switch mechanism, which switches the pressure of oil supplied to components of the engine between a high pressure level and a low pressure level.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-107485